Bale wrappers are used in agriculture to wrap bales with films of impervious material such as plastic, to envelop the baled material in an airtight manner for the process of ensilage of fodder. The plastic is wrapped by rotating the bale around its horizontal axis, while either the bale, as disclosed in GB 2 159 489 A, or an arm carrying the wrapping material, as disclosed in EP 0 829 197 A and EP 1 186 225 A, is rotated simultaneously around a vertical axis. Such wrappers are typically mounted on a common frame with a baler as disclosed in EP 1 186 225 A or embodied as independent machines connected to a tractor for transporting either the wrapper to the respective bales lying on a field or the bales to the wrapper as disclosed in EP 0 829 197 A and GB 2 159 489 A.
In order to prevent accidents, wrappers with rotating arms are usually protected by mechanical sensing arrangements in the form of light secondary arms running ahead of the main arms holding the film dispensers. Once one of the secondary arms comes in contact with an obstacle, it tilts back and activates a braking mechanism that stops the main arms before they hit the obstacle. This safety system is however only reliable within a certain speed range and thus limits the rotational speed of the arms and the working speed of the entire wrapper. When the wrapper is used in combination with a baler, the time required for wrapping the bale is, however, critical for the throughput of the combination.
It has been proposed to provide a self-propelled, unmanned autonomous wrapper with safety devices for avoiding accidents (DE 196 08 014 A). The safety devices comprise mechanical sensing rods and infrared sensors for detecting organisms and engines based upon the generated heat. If, for example, the wrapper comes closer than 20 m to a tractor/baler combination operating on the same field, detected by means of positioning systems, the wrapper is automatically switched off, until this distance is exceeded again. It should however be mentioned that presence sensors like those proposed in DE 196 08 014 A suffer from the disadvantage that they detect all kind of obstacles such as bales lying in the field, trees or fences, and thus often lead to unintended interruptions of work.